adding a "main theme forum"

i just installed my new bbpress forum with wordpress 2.9.2 integration which works fine… now i see how to add topics which is nice, but just can’t find anything about adding “forum main theme”

I am not talking about “design theme” (which by the way you guys gotta change that it’s ridiculous to call forums by “forum main theme” very confusing and almost impossible to find anything that is not related to the actual “design theme”)